Function run_idle_face_task 

pub async fn run_idle_face_task(
    face_states: Arc<DashMap<FaceId, FaceState>>,
    face_table: Arc<FaceTable>,
    fib: Arc<Fib>,
    rib: Arc<Rib>,
    cancel: CancellationToken,
    discovery: Arc<dyn DiscoveryProtocol>,
    discovery_ctx: Arc<EngineDiscoveryContext>,
)

Background task that removes on-demand faces that have been idle for too long (no packets sent or received within IDLE_TIMEOUT_NS).

Runs every 30 seconds until the cancellation token is cancelled.